200 Herbert L. Hawkins — Studies on the Eohinoidea, etc. 



small and so congested with granular ornament that their absence 

 from that region is probably more apparent than real. Their general 

 proportions and distribution are accurately shown in PI. 1, Fig. 3, 

 of GrfiOL. Mag., Dec. VI, Vol. 1, 1914. It is noteworthy that the 

 obliquity of their axes is exactly opposite to that of the adapical 

 tubercles, the areolae being inclined towards the ambitus in each case. 



(e) The Homology of the " Sunken Tubercles" of Discoides and the 

 " Glassy Tubercles" of JEchinoneus. 



The ''sunken tubercles" of D. cylindricus are found over the 

 whole coronal surface of the test except near the apex and on the 

 iuterambulaera of the adoral surface. Their distribution on the plates 

 seems to indicate that they ai % e not fundamentally special structures 

 but are modified examples of the secondary tubercles. As far as 

 can be ascertained, the areolae of the c< sunken tubercles" are 

 generally raised above the surrounding level on the side away from 

 the ambitus, and are sunk more or less deeply below it on the 

 opposite side. The greater part of each areola is depressed, and the 

 suuken part is deeper than the raised part is high. Thus their 

 mamelons, and whatever articulated projections that may have fitted 

 on to them, are inclined towards the ambitus. These structures 

 occur in Plesiechinus, Dyyaster, JTolectypus, and Coenholectypus as 

 well as in Discoides, but, as far as my present observations have 

 indicated, reach their maximum development, at least as regards 

 numbers, in the last-named genus. They seem not to occur in 

 Conulus. The material at present available for a study of Pyrina is 

 inadequate, but I believe that they do occur in F. desmoid insi, 

 though in that species they seem to be wholly sunken, with an axis 

 once more at right angles to the surface of the test. I have not seen 

 any comparable structures in any other Mesozoie genera. 



There are two sets of structures found in Tertiary and Recent 

 forms with which comparison is possible. Many of the more 

 specialized Spatangoids, such as Lovenia and JSupatagus, have a few 

 large tubercles on the interambulacra of the adapical surface. These 

 are set in very deeply sunken areola?, and are usually oblique in 

 relation to the test-surface. But two equally important features in 

 the Spatangoid tubercles are in direct coutrast with those of Discoidfs 

 and its allies. The sunken tubercles of the recent forms are always 

 primaries, iu point of size, while those of Discoides are always 

 smaller than the main tubercles ; and the sunkeu character of the 

 Spatangid areolae seems to be due to the development of secondary 

 thickening on the rest of the plates, and its absence from them, while 

 the actual elevation of part of the areola? in Discoides shows that 

 such a growth has plaved no part in their production. 



The other series of sunken tubercles with which comparison can 

 be made is found in JEchinoneus and Micro petal on. There can be 

 little doubt that, from a phvlogenetic standpoint, these genera are 

 much more nearly related to the Holectvpoida than are the true 

 Spataugids, although it seems equally certain that they are not 

 directly descended from Discoides. Scattered more or less pro- 

 miscuously over the plates of both areas in Echinoneus are the small 



